Tiny iron spheres are oldest fossilised space dust

Japanese
researchers have discovered the first micrometeorites known to land
on Earth. No larger than droplets of fog, the spherical, iron-rich
particles arrived 240 million years ago, 50 million years before
the previous record-holding space dust.

"These are the oldest fossil
micrometeorites I've ever heard of, and the preservation is
fantastic. They look exactly like their modern equivalents," said
geologist Susan Taylor of the US Army Corps of engineers,
who wasn't involved in the work, published in Geology
on 4 May. "If we can figure out where these things came from,
they can help inform us about the history of the solar system."

Meteorites and micrometeorites come from comets and asteroids,
many as old as the Solar System itself. Although larger space rocks are more
popular, they're exceedingly rare. The overwhelming majority of
extraterrestrial material is dust, of which some 30,000 tons falls
from space each year.

About 90 percent vaporises while passing through the Earth's atmosphere,
producing the sparks seen during meteor showers. Of what makes it
to the ground, a small fraction gets stuck in mud, clay and other
sediment that becomes fossilised.

By finding and comparing rare collections that span the geologic
record, researchers can make guesses about cosmic conditions that
Earth experienced throughout its history. Trouble is, the farther
back in time geologists go, the harder it is to find well-preserved
micrometeorites.

"I gave up trying to find them years ago," said Taylor. "These
aren't tough particles. They're fragile. Just a little acid etches
out their glass, causing them to disintegrate. It's amazing we find
ones this old at all."

Geologist Tetsuji Onoue of Kagoshima University
discovered the new specimens in ancient shale and chert rock from
Ajiro Island, at the southern tip of Japan.

To get to the roughly 300 micrometeorites, Onoue's team crushed
and sieved the rock, cleaned it with ultrasonic waves, then ran a
magnet over the material. Under an electron microscope, they found
10-micron-wide spheres that somehow survived descent through
Earth's atmosphere, chemical weathering and 240 million years of
punishing fossilisation.

The samples are not representative sample of ancient space dust
-- everything but the iron-containing spheres dissolved away -- but
Taylor said they still offer a unique lens into cosmic history.

"We spend a lot of money and time sending things out into space,
which is great, but here on Earth we have tens of thousands of tons
of extraterrestrial material arriving each year," she said. "If we
looked for more of them, we could get more information about comets
and asteroids throughout time without having to visit them."